The present invention relates to the art of polybenzazole polymers and membranes containing them.
Gas-separation membranes separate mixtures of gases by permitting one gas in the mixture to diffuse through the membrane from a region of high pressure to a region of lower pressure at a faster rate than another gas in the mixture. The result is an enriched stream of the faster diffusing gas on the low pressure side of the membrane and an enriched stream of the slower diffusing gas on the high pressure side of the membrane. Gas separation membranes may be isotropic, in which the entire membrane is a dense polymer traversed by solution-diffusion mechanisms, or they may be asymmetric, in which a dense polymer skin is attached to a porous layer of the same polymer, or they may be composite membranes in which a dense layer of one polymer is attached to a porous layer of a different polymer. In asymmetric and composite membranes, the porous polymer acts to support the membrane and possibly to filter out non-gaseous impurities, but it does not ordinarily assist in the separation of gases. That function is performed by the dense skin of an asymmetric membrane or the dense discriminating layer of a composite membrane.
The separation properties of a gas-separation membrane are typically characterized in terms of permeability, flux and selectivity. The permeability of a membrane with respect to a particular gas is measured in units of Barrers (Ba). One Barrer is defined as: ##EQU1## The flux of the membrane is the speed at which a particular gas crosses the membrane. It is equal to permeability divided by membrane thickness. A standard flux unit is typically: ##EQU2## Selectivity is the permeability or flux of the faster permeating gas divided by the permeability or flux of the slower permeating gas. It is a unitless measurement.
It is desirable that a membrane have both high selectivity and high flux with respect to the gas or gases which selectively permeate through the membrane. However, it is frequently found that modifications in a membrane which increase selectivity also decrease flux, and vice-versa. It is also desirable for the membrane to possess sufficient strength to form thin membranes which can be handled and used without tearing. Additional desirable characteristics include increased high-temperature stability and resistance to common organic solvents, contaminants and vapors.
What are needed are polymers having two or more qualities of high permeability, high flux, high strength, high continuous use temperatures and/or high resistance to common organic solvents and vapors.